Threshold Behavior of Quasi-Ballistic Transport in Ultrafast Depletion-Field Dynamics at a GaAs Subsurface

Ultrafast dynamics of subsurface depletion fields play a central role in carrier separation and transport at semiconductor surfaces, yet the transport mechanisms governing their temporal evolution remain insufficiently constrained experimentally. Here, we identify a threshold behavior of quasi-ballistic transport in the ultrafast depletion-field dynamics at a GaAs subsurface, revealed by time-resolved electric-field-induced sum-frequency generation spectroscopy. By continuously tuning the pump wavelength, we observe a pronounced, step-like increase in the field recovery time once the excess energy of photoexcited carriers exceeds the Γ-L valley separation. This behavior defines a transport threshold at which intervalley scattering becomes competitive with, and ultimately faster than, field-driven carrier sweep-out across the depletion region, thereby giving rise to a scattering-limited space-charge reorganization regime. These results establish a direct and experimentally accessible criterion for identifying transport-regime transitions in ultrafast subsurface dynamics and highlight the capability of electric-field-sensitive nonlinear spectroscopy to resolve mechanismlevel changes in carrier transport.